Loudness meter



' April 24, 1962 BOLSTON I 3,()3152-8 LoU'DNEss METER Filed Nov. I, 1960 2 sheets-sheet 1 /A 2yIfUNOTIO I N ET @'75 CPS GENERATOR 7 75 l5OCPS 1 IFUNCTION 27'] GENERATO Jy I s *l I lEUNCTION 7 T/Fvo @30CPS GENERATOR 77T i A I y /f/ GOOCPS FUNCTION n Y 500 GENERATOR if COMBININQJDICATOR omgoocps FUNCTION {gf- CRCUT i; GENERATOR 41H r FUNCTION *TjlQ-OO 2qOOCFSGEII'I-:RATOIEI 77 r f.. ,1. u FUNCTION MAXIMUM SELECTOR 1N VENTOIL Aprli 24, 1962 A L BOLSTON 3,031,528

LOUDNESS METER 'y Filed Nov, 1., 1960 v v r sheets-sheet 2 4 zzfooao)"*g2 40d (48009609) l l( OO l 2o 3o 4o 5o eoo ao so loo no VSOUND PRESSURE LEVEL IN DB .r4/i i l yf' OUTPUT VOLTAGE LOU DN ESS IN VENTOR.

NPUT VOLTAGE I (SOUND PRESSURE) Wd/M United itares iiatent @ffice A aesiszs This invention relates to apparatus for detecting sound pressure and indicating the` coirespondingiloudness.

Noise can easily be measured by detecting sound pressure levehbut this type of measurement-has little meaning since the human reaction to noise is not linearlyv related to sound pressure. The loudness or apparent magnitude ofvvarious sounds can be denedby. empiricallyderived functions which were established by experiments with large numbers of persons. Such experiments are described in an article by S. S. Stevens in the September 1957 issue of the publication Noise Control, pp. 11--22. It has been found that loudness varies as a function of sound pressure according to a different relationship for the various frequency bands in the audio spectrum. Also, when noise having amplitude peaks at several spaced frequencies is present, then the total loudness can he defined by a certain relationship between the loudness n the band having the maximum sound pressure and the loudness in the remaining bands. One way'of expressing this relaonship is .according to a function: ST=SM+O.3(S-SM), where ST is the total loudness, SM the maximum loudness from the several frequency bands in the audio spectrum, and S is the sum of the loudness in all of the frequency bands.

It is presently proposed to provide apparatus for producing a reading of total loudness wherein the sound in question contains components in several frequency bands.

In accordance wth this invention, 'a pickup is utilized to detect sound pressure to produce a signal corresponding thereto. This signal is fed to a plurality of lters corresponding to the frequency bands in the audio spectrum. The signal output from each filter, related to the sound pressure in is particular band, is applied to a function generator which produces an output corresponding to the subjective loudness characteristic for this particular band. The maximum loudness signal appearing on the various frequency bands is detected, and this maximum signal is combined with the sum of .all the loudness signals according to a certain relationship. The combincd signal is indicated, in units of sones if desired,

providing a reading of the loudness of a complexnoise.

In the drawings: FIGURE l is a block diagram of loudness measuring7 apparatus incorporating the principal features of the invention;

FIGURES 2 and 3 are schematic diagrams of portions of the apparatus of FIGURE -1;

FIGURE 4 is a graphic representation in logarithmic cfs-ordinates of the subjective loudness characteristics of the various frequency bands utilized in the apparatus of- FIGURE 1;

' FIGURE 5 is a schematic diagram of one of the func- This A C. output is applied separately t0 1 the inputs of eight octave band filter circuits 11-18 which 2 filter circuit is applied to the input of one of a plurality ci function generators 21.4.3, each of which produces an ou-:put signal at one of a set or' output terminals 311-38 which is related to sound pressure by the subjective loudness characteristic for the particular frequency band. Each of the output terminals 3x1-3S is connected separateiy through one of a piurality of diodes 41-48 to a common line 49 which is applied to the input of a maximum selector or voltage iscriminator 50. The maximum seiector 5) is adapted to detect the signal of greatest magnitude and to provide an inverted signal outputcorresponding to this maximum signal. The output terminals 33.--33 are further connected to a common line and to an input 52 of a combining circuit 53 which also receives the output of the maiimum selector 50. This combining circuit 53 produces an output related according to a certain function, such as the mathematical expression set i forth above, to the sum of the function generator outputs and to the maximum Qiector output. The combined output from the circuit 53 is applied to an indicator 54 which provides an outnut reading of loudness.

With reference to FGURE 2, the filter circuit and func.-

tion generator for one of the octave band channels of FIGURE 1 are shown in more detail. More specifically, the fil er circuit 1]. for the lowest frequency channel is illustrated along with the o ation generator 21 which is adapted to produce an ou "ut voltage related to loudness winin this frequency b.. d. 'Ihe filter circuit 11 includ-es an input terminal 68 which is coupled to the microphone 10 through suitable pre-amplifiers, if necessary. A potentiometer 61, is connected across the input for calibration purposes, and a tapped portion of the output of this potentiometer is connected through an amplifier 62 to a bandpass filter 63. This filter is responsive only to the lowest frequencies of the audio spectrum, designated as 0-75 cycles er second. The output of the iilter -53 is applied through an amplifier 64 to a' full-wave detector circuit 65, producing a D.C. output corresponding to the magnitude of :he A C. signal produced by the microphone 10 within t're frequency range of the filter 63. The DC. output is aoolied to the function generator 2l which includes a voltage divider arrangementincluding several resistors 66, 67, 68, and 69. The output terminal 31 of the function generator 21 is connected across a tapped portion of the variable resistor 69. The junction point of the resistors 67, 63 is connected to a negative voltage source 70 through a Zener diode 71 sct forth above. 'The output terminals 31-38 of the function generators are shoxm connected together through essentially cover the audio spectrum. The output of each a plurality of like resistors 73 to a common line or to the input S2 of the combining circuit 53. the terminals 31-33 are also connected through a plurality of diodes 41-48 to a common lire t9 which in turn is connected through an input resistor 74 to an input 7S of a summing amplifier 76.l The amplifier 76 is connected to an output line 77, and this outputl is coupled through a feedback resistor 78 to an input 79 of the amplifier. The vaines of the resistors 74 and 78, and the remaining circuit components, are selected such that die overall gain of the maximum selector S0 is unity, resulting in an output on the output line 77which is equal in magnitude to the maximum function generator output, vbut is inverted or negative. The output line 77 is applied through an input resistor to an input 80 of a second summing ampli- Patel-asd Apr. 24, 1962 tier 81 in the combining circuit 53. The output of this v amplifier is applied to an output line 82, and a portion of the output is coupled back through a feedback resistor 83 to another input 84 of the summing amplifier; A common terminal of the resistors 73 also appears as an input 52 to the amplier 8L 'Ihe values of the input and feedback resistors for the stage including the 'amplifier S1 are selected to provide an overall gain of 0.3, or for to an input 85 of a further summing amplifier 86. The

output of the maximum selector 50, appearing on the output line 77, is likewise applied through an input resistor to an input 87 of the amplifier 86. The output 38 of this amplier is coupledv back through a feedback resistor 89 to another4 input 90. Since both the combined signal and the maximum` signal rave. eeen inverted, then the output appearing7 onthe line SS will be the sum of thc maximum 'and combined signals, and this output is applied to the Aindicator 54 which may ne any suitable type ofvoltage-responsive meter or recording means.

With reference to FIGURE 4, there is shown a graphic representation of loudness, in units of sones, plotted as a function of sound pressure level, in units of decibels above a reference level, for the eight octave bands in the audio spectrum as utlizcd in this invention. These characteristics were derived through experiments as set forth in the above-mentioned article by S. S. Stevens. A line 91 represents subjective loudness as a function of sound pressure for the lowest band or -75 cycles, correspondiug to the channel including the lter 11 and the function generator 21. The function getrator 21 necessary lo reproduce this curve 91 is shown in detail in FIGURE 2. Likewise, a line 92 corresponds to the octave band of 75-150 cycles and represents the characteristics of the channel including the filter circuit 12 and function generator 22 A line 93 corresponding to 150- 300 cycles is the basis for the transfer characteristics of the channel including the lter circuit 13 and function generator 23. The two channels including the filter cir cuits 14, and the function generators 24, 2S corresponding to the octave bands G-650 and 60G-1200 cycles, both have similar transfer characteristics which are represented by a line 94. A specific example of the type of function generator necessary to provide a transfer characteristic resembling the line 94 is explained in detail with reference to FIGURES 5 and 6. The transfer characteristcs of the channels including the lter circuits 16, 17, and 18 and the function generators 26, 27, and 28 are represented in FIGURE. 4 by lines 96, 97, and 98, respectively. These channels correspond to the three octave bands between 1200 and 9600 cycies.

With reference to FIGURE 5, there is shown a function generator corresponding to the function generators 24 and 25 of FIGURE l which are necessary to develop outputs related to sound pressure according to the line 94 of FIGURE 4 which, in linear coordinates of output voltage vs." input voltage, may be represented by a line 100 of FIGURE. 6m. The'function generator 25 includes input terminals 101, whichl are connected to the output of the generator circuit 15, and a voltage divider including fixed and variable resistors 102 and 153 connected by a line 104 to another set of variable resistors 105 and 106. The output terminal of the function generator is connected to a variable tap on the resistor 106. To obtain a series of break points 107-111 as shown in FIGURE 6, a plurality of shunt circuits are connected to the common line 104, as shown in HGURE 5. These shunt circuits include a plurality of Zener diodes 113-117, each being connected to a negative voltage source r'112 through one of a set of series resistors 118-122. As plotted in FIGURE 6, the output v'oltage appearing at the terminal 35 is initially related to the input voltage across the terminals 101 by a linear func-A tion determined by the simple divider circuit comprising resistors 102, 103, 105, and 106. However, when an input voltage corresponding to a point 107 is reached,

the lZener diode 113 breaks down. This vprovides the vshur across the divider circuit, and reduces the slope of the transfer characteristic. The characteristic will then follow a linear. function until the break point 103 is reached, at which time the Zener diode 114 will break down,y providing an additional shunt across the output of the voltage divider. It is seen that within limits virtually any function can be approximated in this manner by adjusting the variable resistors 103, 105, 106, and 118122. The function generators 22, 23, 26, 27, and 2S necessary to reproduce the functions defined by the lines 92, 93,

96, 97,v and 98 of FIGURE 4 are of the same form as l In the operation of the apparatus described above, Veach Y of the einht octave channels willV produce a D.C."output voltage, at the respective' one'of the terminals 31-38,

which is related according to the subiective loudness function specified in FIGURE' 4 to the sound pressure in the particular frequency-band. The maximumof the outputs appearing on the terminals 31-3S will appear on the line 49 since only one of the diodes 41-48 will con.- duct at any one time. For example, if the output at the terminal 33 is greater than any of the other outputs, then the diode 43 will conduct, the voltage on the line 49 will equal the voltage at the terminal 33, and this will prevent any of the remaining diodes from conducting. This maximum voltage will appear inverted on the line 77. The sum of all of the outputs will appear on the input 52 as an input to the amplifier 81, but since the maximum signal appears inverted at the input 80, the output on the line S2 will be a fraction of the sum of the .outputs minus the maximum, or will equal the function 0.3(S-SM). This output will appear on the input 85 of the summing amplifier 86 in an inverted form'while the SM signal will appear on the input 87. Thus the output appearing on the line SS-will 'oe related to the function S1,5-}-0.3(S-SM). This signal is indicated on the indicator 54, producing a direct reading of loudness which may be calibrated in units of sones.

While this invention has been described in terms of an illustrative embodiment, it is of course understood that `to produce a signal related to sound pressure,l signal translating means connected to said pickup means to receive said signal, said signal translating nteans being adapted to produce a plurality of output voltages each related by one of a plurality of functions to sound pressure Vin one of a plurality of frequency bands, each of said plurality of functions corresponding to the subjective loudl ness characteristic for the particular frequency band,

. discriminating means connected to said signal translating means to receive all of said plurality of output voltages and adapted to produce a maximum output corresponding to the maximum thereof, combining means connected to' said signal translating means to receive said output voltages and also connected to said discriminating means to receive said maximum output, said combining means bcing adapted to produce a combined output related by a given function to said electrical outputs and said maximum output, and indicating means connected to said combining means to receive said combined'out'put.

2. A oudness meter comprising a sound pickup adapted to produce a signal related to sound pressure, a plurality of translating circuits connected to said pickup to receive output lfrom said translating circuits, combining means connected to the outputs of all of said plurality of translating circuits to receive all of said electrical outputs and also connected to said discriminating means to receive said maximum output, said combining means being efiective to produce avcombined output related by a predetermined kfunction to said electrical outputs and said maximum output, and indicating means connected to said combining means to receive said combined output and adapted to produce an indication thereof.

3. A loudness meter comprising a sound pickup adapted to produce a signal related to sound pressure, a plurality of octive tilters connected to said pickup to receive said signal, each of said lters being adapted to produce an output voltage related to sound pressure in an octave band, a plurality of function generators, each of said function generators having an input connected to one of said lters to receive said output voltage and being adapted to produce an electrical output related to said output voltage by a specified function, discriminating means connected to the outputs of all of said plurality of function generators to receive all of said electrical outputs and adapted to produce a maximum output corresponding to the maximum thereof, combining means connected to said plurality of function generators to receive said electrical outputs and also connected to said discriminating means to receive said maximum output, said combining means being adapted te produce a cornbiued output related by a given function to the sum of said electrical outputs and said maximum output, and indicating means connected to said combining means'to receive said combined output.

' 4. A loudness meter comprising a sound pickup adapted to product a signal related to sound pressure, a plurality of octave lters connected to said pickup to receive said signal, each of said filters being adapted to produce an output voltage related to sound pressure in an octave band, a plurality of function generators, each of said function generators having an input connected to one of said lters to receive said output voltage and being adapted to produce an electrical output related to said output voltage by a specified function, discriminating means connected to the outputs of all of said plurality of function generators to receive all of said electrical outputs and adapted to produce a maximum output corresponding to the maximum thereof, combining means connected to the outputs of all of said plurality of function generators to receive all of said electrical outputs and also connected to said discriminating means to receive said maximum outputin a subtractive relationship and effective to produce a combined output, and indicating means connected to said combining means and to said discriminating means to receive said combined output and said maximum output and adapted to produce an indication related to the sum thereof.

' bands r:nd being adapted to produce an electrical output related to -said portion of said signal by a specified function, discriminating means connected to tlae outputs of all of said plurality of translating circuits to receive all yofi n 'I said electrical outputs and adapted to produce a marti-k mum output corresponding to the inverse of the maximum electrical output from said translating circuits, rst combiningmeans connected to tbe outputs of all of said plurality of translatingv circuits to receive all of said f electrical outputstand also connected to said discriminat ing means to receive said inverted maximum output and etective to produce a combined output corresponding to the inverse of a predetermined fraction of the snm'of the electrical outputs minus the maximum output, second combining means connected to said first combining means and to said discriminating means to receive said inverted' combined output and said inverted maximum output and adapted to produce an output voltage corresponding to the sum of t'ne maximum output plus the predetermined fraction of the sum of the electrical outputs minus the maximum output, and indicating means connected to said second combining means to receive said output voltage and adapted to indicate the magnitude thereof.

6. A loudness meter comprising a sound pickupadapted to produce a signal related to sound pressure, a plurality of octave filters connected to said pickup to receive said signal, each of said filters being adapted to produce an output voltage related to sound pressure in one of a plurality of adjacent octave bands in the audio spectrum, a plurality of function generators, each of said function generators having an input connected to one of said filters to receive said output voltage and being adapted to produce an electrical output related to said output voltage by a function which corresponds to the subjective loudness characteristic for '.'ne octave band, discriminating means connected to the outputs of all of said plurality of function generators to receive all of said electrical outputs and adapted to produce a maxi-mum output corresponding to the inverse of the maximum electrical output, first combining means connected to the outputs of all of said plurality of function generators to receive all of said electrical outputs and also connected to said discriminating means to receive said inverted maximum output and effective to produce a combined output corresponding to the inverse of a predetermined fraction of the sum of the electrical outputs minus the maximum output, second combining means connected to said first age corresponding to the sum of the maximum output plus the fraction of the vsum of the electrical outputs minus the maximum output, and indicating means conected to said second combining means to receive said output voltage and adapted to indicate the magnitude thereof.

' No references cited. 

